As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2010.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
In the production of motor vehicles in particular aluminium alloys of the AA5xxx- and AA6xxx-series alloys like 5051, 5182, 5454, 5754, 6009, 6016, 6022, and 6111, are common and proven alloys to produce automotive structural parts and body-in-white (“BIW”) parts.
Body-in-white consists of the structural components of the automobile, not including closures and moving parts (e.g. door panels, hood panels, trunk lid panels). Other automotive structural parts are for example B-pillar, tunnel section or engine bulkhead, all of which may be manufactured by the inventive method. During the last 7 to 12 years, (very) high strength steel grades were developed and different production, forming and joining techniques were created and setup which allowed car manufactures to use these grades to produce automotive structural parts and body-in-white (“BIW”) parts. Using these grades allows the car manufactures to achieve high passenger safety in case of crash while adding (much) less weight as would be the case when traditional steel grades and AA5xxx and AA6xxx-series aluminium alloys would be used for this purpose.
To be able to achieve even higher weight savings, there is a demand for the use of high strength aluminium alloys, in particular for formed structural and BIW parts, which are formable and having in particular increased strength after being subjected to a paint bake cycle. In addition, the material properties normally required to produce such parts include a high formability for the forming operation (typically by means of stamping, deep drawing, or roll forming), high mechanical strength after paint baking so as to enable down gauging and thus minimising the weight of the part, good behaviour in the various assembly methods used in motor vehicle manufacturing such as spot welding, laser welding, laser brazing, clinching or riveting, and an acceptable cost for mass production.
AA7xxx-series aluminium alloys are able to deliver the required strength after being subjected to a paint bake cycle. However, formability of these alloys is poor to very poor, in particular because AA7xxx-series alloys naturally age after solution heat treatment followed by quenching (together referred to herein as “SHT”), leading to a significant strength increase by about 80% within one week after solution heat treatment and quenching. Formability would be acceptable if AA7xxx-series sheet products could be formed within a few hours of SHT, but this is not practical because the solution heat treated and quenched sheet product needs to be transported from the rolling mill to the automobile manufacturer before it is formed into its final shape by the latter.
To overcome this problem, it has previously been suggested in DE 10 2010 045 025 A1 and WO 2010/142576 to effectively repeat solution heat treatment and quenching immediately prior to forming, i.e. on the site of the automobile manufacturer. This method is referred to as “W-temper process”, because the sheet material is effectively returned to the W-temper (as explained below). According to WO 2010/142576, the W-temper process involves soaking the sheet product for a period of 3 sec to 15 min. at a temperature between 400° C. and 490° C., followed by rapid cooling or quenching, namely water quenching (or quenching via contact between cold plates).
A disadvantage of the W-temper process is the high energy consumption required for the high temperature treatment, as well as the necessity of water quenching. In addition, the higher the temperature, the higher the risk that the aluminium alloy sheets stick to the tooling if direct contact heating is used. Further, one has to be careful not to start artificial aging (e.g. by paint baking) too early after the W-temper process, as this may lead to reduced mechanical properties, thus requiring unwelcome delays in the automobile production process.